Several Things. High HDL levels may be the result of genetics (hyperalphalipoproteinemia), exercise, increased alcohol intake, or medications (niacin, fibrates). Rare causes include obstruction in the biliary tract, very high blood levels of dilantin, and organophosphate toxicity.

Many things... First, elevated HDL is a good thing. This helps protect your body against the damage done by LDL (or the "bad" cholesterol). Genetics, exercise, healthy diet all can explain an elevated hdl.